In a typical cellular communications system, wireless user equipment units (UEs), for example, mobile phones, communicate via a radio access network to one or more core networks. A radio access network covers a geographical area which is divided into cells, with each cell area being served by a radio base station. Several base stations are connected, typically via land lines, to a control node known as a radio network controller (RNC). Such a control node supervises and coordinates various activities of the several radio base stations which are connected to it. The radio network controllers are typically connected to one or more core networks. One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation (3G) system and UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units. Fourth generation systems are evolving towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network.
In many radio access networks the radio base station is a concentrated node with most of its components being located at a concentrated site. However, a radio base station can also be configured with a more distributed architecture. For example, a distributed radio base station can take the form of one or more radio equipment (RE) portions that are linked to a radio equipment control (REC) portion over an internal interface. One example of an internal interface of a radio base station which links a radio equipment portion of the radio base station to a radio equipment control portion of the base station is the Common Public Radio Interface (CPRI). The Common Public Radio Interface (CPRI) is described in Common Public Radio Interface (CPRI) Interface Specification Version 4.1 (18 Feb. 2009) and also Version 4.2 (2010) and Version 5 (2011).
The Common Public Radio Interface (CPRI) is an industry cooperation aimed at defining a publicly available specification for the key internal interface of radio base stations between radio equipment control (REC) and radio equipment (RE), thereby allowing base station manufacturers to share a common protocol and more easily adapt platforms from one customer to another. In essence, a radio base station is decomposed into two separate blocks, known as REC and RE. The REC provides access to a UMTS network, for example, via the lub interface, whereas the RE serves as the air interface to user equipment, known as Uu in a UMTS network. The REC generally comprises the radio functions of the digital baseband domain, whereas the RE contains analogue radio frequency functions. The functional split between the REC and RE is done in such a way that a generic interface, CPRI, based on In-Phase and Quadrature (IQ) data can be defined. Several IQ data flows can be sent over one physical link with each data flow reflecting the data of one antenna for one carrier, the so-called antenna carrier “AxC.” Several AxC's having the same sampling rate may be aggregated into an “AxC Group.” IQ data of different antennas along with control data are multiplexed onto a transmission line. The CPRI has a basic frame structure for carrying a control word and an IQ data block. An “AxC Container” is a sub-part of the IQ data block of one basic frame. For example, an AxC Container for E-UTRAN contains one or more IQ samples for the duration of one UMTS chip or it may contain IQ sample bits and stuffing bits.
In addition to IQ data, the CPRI interface supports control and management (C & M) information which is exchanged between the control and management entities within a REC and a RE. The control and management data are either sent as inband protocol or by layer 3 protocols that reside on top of appropriate layer 2 protocols. C&M data are time-multiplexed with synchronisation data and the IQ data over the CPRI. Two different layer 2 protocols for C&M data are supported by CPRI. These are Ethernet and High-Level Datalink Control (HDLC). A vendor-specific channel may also be supported. HDLC is a protocol developed by the International Organisation for Standardisation and falls under ISO standards ISO 3309 and ISO 4335 HDLC is sometimes referred to as the slow C&M channel in CPRI (Ethernet being referred to as the fast channel) HDLC can support several data rates and operates to provide a reliable communications path between nodes with acknowledged data transfer.
The functional split between the REC and RE allows the RE to be positioned close to an associated antenna. This reduces the distance which the associated signals have to travel before they are received by the RE, thereby negating the need for tower-mounted amplifiers and antenna system controllers. The link between the RE and REC is generally optical, allowing the link length to be much greater when compared with wired coaxial systems. Therefore, the distance between the RE and RRC can be up to 40 Km, thereby increasing the flexibility of deployment of RE's within the network when utilising CPRI. One REC may be linked to two or more REs in a chain topology with each RE being configured to forward data to other REs in the chain. Similarly, an RE may be linked to multiple REC's in a chain topology with each REC being configured to forward data to other REC's in the chain.
Many of the functions which an REC has to perform, which include channel coding/encoding, spreading/despreading, frame and time slot generation, for example, may be realised by a proprietary digital signal processing device. Two examples of such DSP devices which support the CPRI are the Freescale B4860 and the Freescale MSC8157 Broadband Wireless, Access Six Core DSP which is described in Freescale Semiconductor Data Sheet MSC8157E, November 2011. This Digital Signal Processor includes (inter alia) a CPRI unit which includes a “framer” module which handles the transmission and reception of all IQ data, and a DMA (direct memory access) module associated with the framer which transfers the receive and transmit IQ data to and from antenna carrier buffers in a memory. The framer is provided with an auxiliary interface so that chain topologies can be supported. A typical CPRI unit contains receive and transmit configuration tables that determine which AxC's in a frame to transfer to or fetch from system memory and their location in the basic frame or hyperframe. For daisy chain configurations, each CPRI unit may utilise receive mask registers with the masking of bits of the auxiliary interface. For example, if a masking bit is “set” then data coming from the framer of a second CPRI unit is sent to a subsequent RE or REC in the chain (sometimes referred to as “forward mode”). Conversely, if the masking bit is “cleared” then the data read from the system memory is sent to a subsequent RE or REC in the chain (sometimes referred to as “local mode”). The auxiliary mask is therefore only used in daisy chain configurations.
Current CPRI controllers define the allocation of AxC per each REC pseudo-statically, that is, the IQ allocation for the different RECs in a chain is static, and any change takes approximately 10 ms. Reconfiguration of the IQ allocation involves both the REC-chain and the RE-chain, that is to say, that the RE-chain reconfiguration is mutually dependent on the REC-chain reconfiguration. Usually, CPRI can transfer larger BW than one device can process which is why REC chaining is used whereby each REC processes part of the data. However, actual loading of a communications cell can vary. For example, a cell may be much less loaded during the night time than during the day so that in some circumstances fewer AxCs would require processing during a particular operational period. Therefore, it would be advantageous to be able to dynamically reconfigure the allocation of AxCs between chained RECs so that in quiet periods, one or more RECs could be powered down thereby reducing overall power consumption of the base station system.